1. Field of the Invention
The present invention relates to a control system for controlling the timing of ignition of an internal combustion engine.
2. Description of the Prior Art
It is generally known that an internal combustion engine provides the highest efficiency concerning the output power and fuel economy when the engine is operated in a state of small knocking (critical state of knocking).
Among conventional control systems of this type there has been known a system for controlling the ignition timing to the critical state of knocking on a feedback basis by delaying the ignition signal for a certain crank angle or in proportion to the degree of strength of knocking from an ignition timing as determined based on the predetermined ignition timing characteristics at each occurrence of knocking and by decreasing the delay of ignition timing at a certain time constant after knocking has ceased. This system, however, causes all cylinders of engine to have an equally delayed ignition timing in order to suppress knocking, and if knocking occurs in one cylinder, the ignition timing is delayed not only for that cylinder, but for the remaining cylinders by the same amount of the crank angle.
Generally, each cylinder of engine has different nature on the occurrence of knocking, i.e., different ignition timing for the critical state of knocking, due to the variations of the structure, component parts and distribution of mixture. On this account such a conventional control system sets up the ignition timing to the critical knock point of a cylinder which most often tends to knock. This not always provides the best ignition timing control for the engine, and does not achieve ignition for all cylinders at their individual critical knock points.
To cope with this problem there have been proposed several ignition timing control systems in which the delay of ignition timing is controlled on a feedback basis for each cylinder individually.
Any of the foregoing feedback control systems for suppressing knocking is based on the control of delay of the ignition timing with respect to the reference ignition timing in the crank angle, and for this purpose the reference ignition timing must be set to an advanced crank angle preceding the critical knock point. On this account the ignition timing at the beginning of control is always placed within the domain of knocking, and a severe knocking occurs when the control is initiated. Therefore, it is desirable to set the reference ignition timing at a point slightly preceding the critical knock point. However, when the variation of critical knock points among cylinders is taken into consideration, such a critical setting of the reference ignition timing is practically impossible, and it is inevitable for the reference ignition timing to be set more delayed with respect to the point of critical knock point in a certain operating condition. During this operating condition with an excessive delay in the reference ignition timing, the engine operates under the ignition timing which lags behind the optimal ignition timing in the critical state of knocking, and it is not possible to control the ignition of all cylinders to the optimal ignition timing in the entire operating condition.
In addition, the conventional control systems use the reference ignition timing invariably even if the operating condition of the engine has varied, resulting in a significant time lag before the control value converges to a new reference value for the operating condition following the variation, i.e., unsatisfactory response against the variation of the operating condition. Moreover, the control systems are required to have a wide control range in order to cover the entire operational region in which knocking is to be suppressed, and it is difficult to control the ignition timing precisely in the entire operating range.
The occurrence of knocking is greatly dependent on various factors of the operational characteristics of engine, particularly the air-fuel ratio, the temperature of intake air and the humidity of intake air at the time of ignition. Among those, factors of spontaneous conditions such as the temperature and humidity of intake air vary very slowly in a period of a day or year, and therefore the occurrence of knocking also varies in a long period. In other words, during a constant operating condition, knocking will occur at a relatively constant frequency and degree of severity when observed in a short term. Namely, the control value needed to suppress knocking which occurs in a constant operating condition is substantially constant in a short term. Accordingly, the ignition timing can be controlled to the critical knock point, while suppressing the occurrence of knocking precisely and in high response by controlling the engine using the previous control value during a constant operating condition stated by specific operational parameters, and by carrying out the sequential modification control with a narrow control range in response to each knock signal for the occurrence of a small knocking during the above control. For the spontaneous factors which vary in a long period as mentioned previously, modification is possible by progressively varying the amount of above modification control value.